Conventionally, use has been made of, as a method for removing impurities (e.g., solvents, monomers, oligomers, and by-products) remaining in a polymer or a rubber, such means as evaporation by heating and vacuum evaporation. However, small amounts of impurities still remain, and some of these impurities deteriorate the quality of the product, or are harmful to the human body. If the polymer or the like is heated in order to reduce its impurity content, the polymer will be, for example, changed in quality or softened, so that it is impossible to lower the remaining amounts of impurities to below a certain degree.
Therefore, recently, high-pressure extraction processes using for example supercritical fluids have been suggested (e.g., Chem. Ing. Tech. 56 (1984) Nr. 11, S. 856). Japanese Patent Application (OPI) No. 115419/1982 discloses a process wherein cyclic oligomers in a polymer are removed by the use of supercritical fluids.
However, according to the supercritical fluid extraction described in Japanese Patent Application (OPI) No. 115419/1982, cyclic oligomers still remain to a certain degree in the polymer. When impurities are removed by extraction from a polymer by using a high-pressure extraction process, if the polymer is, for example, in the form of a powder or pellets having a small particle size, the volume of the polymer increases due to the absorption of the extractant when the polymer is subjected to the extraction, which sometimes causes the solid particles to stick to each other. Further, in some cases, a change such as a softening or melting of the polymer in the form of powder, pellets, or the like, which is caused under extraction conditions, causes the particles or pellets of the polymer to stick to each other. That is, in conventional high-pressure extraction processes, when a polymer is subjected to extraction, the particles or the pellets of the polymer stick to each other during the progress of the extraction process to form a mass of the polymer. This raises a qualitative problem with the polymer product, is unfavorable when the product is to be removed from the extractor, and raises the necessity of step, such as grinding of the product.
On the other hand, when the temperature is lowered, the extraction speed becomes low, and the extraction requires a longer period of time, which is economically disadvantageous because the apparatus must be made large and the amount of the extractant must be increased.
In particular, when the solid material (e.g., a polymer) to be treated is a powder of small particle size, sticking and the formation of a mass is more liable to take place, and extraction to a high degree is impossible.